The catalytic production of polyethylene is very well known in the art. A very special class of polyethylene is ultra high molecular weight polyethylene (UHMWPE) with a very high average molecular weight ranging from about 1000000 to well above 6000000 grams/mole whereas high density polyethylene (HDPE) typically has a molar mass between about 50000 and 300000 g/mol. Therefore, these linear polymers have an average molecular weight much higher than that of linear high density polyethylene. The polymer synthesis to obtain UHMWPE is disclosed in Journal of Macromolecular Science Part C Polymer Reviews, Vol. C42, No 3, pp 355-371, 2002. The higher molecular weight gives UHMWPE the unique combination of characteristics making it suitable for applications where lower molecular weight grades fail. The very high molecular weight of this polyethylene results in excellent properties for example a very high abrasion resistance, a very high impact resistance, a very high melt viscosity and a low dynamic coefficient of friction. Because of the high molecular weight and the high melt viscosity specialized processing methods like compression moulding and ram extrusion are applied. Due to the high molecular weight, UHMWPE displays bad flow ability when molten, it is difficult to mould it into a pellet form and the product has to be delivered in a powder form and even more important, it has also to be processed from powder. Consequently, the powder properties heavily determine the production process as well as the converting process. For example, this powder has to be stored and to be transported, and consequently the bulk density of the UHMWPE powder is very important. A higher bulk density may decrease clogging at its transportation and it is possible to increase a storable amount per unit volume. By increasing the bulk density, the weight of the UHMWPE per unit volume present in a polymerization vessel can be increased and the concentration of the UHMWPE powder in the polymerization vessel can be enhanced. Similarly in the processing of UHMWPE a high bulk density also is required. As mentioned, typical processing procedures are ram extrusion and compression moulding. Both methods in principle involve sintering of the powder particles (Stein in Engineered Materials Handbook, Volume 2: Engineering Plastics, ASM International 1999 page 167-171). In order for this sintering to become effective, it is very important that a dense polymer powder packing is achieved, which translates into a high bulk density. The bulk density of UHMWPE should be above 300 kg/m3 and even more preferably above 350 kg/m3. Furthermore the average particle size of the UHMWPE powder is an important characteristic. The average particle size (D50) is preferably lower than 250 micrometers, more preferably below 200 micrometers. In addition, the particle size distribution, commonly known as the “span”, defined as (D90−D10)/D50, should be low, preferably below 2, and even more preferably below 1.5.
The shape of the polymer powder particles is translated from the shape of the catalyst particles, also known as the replica phenomenon. In general, when this replication takes place, the average particle size of the polymer is proportional to the cube root of the catalyst yield, i.e. the grams of polymer produced per gram of catalyst. See for example Dall'Occo et al, in “Transition Metals and Organometallics as Catalysts for Olefin Polymerization” (Kaminsky, W.; Sinn, H., Eds.) Springer, 1988, page 209-222. Due to this proportionality, small polymer particles may be produced by reducing the catalyst yield, but this causes high catalyst residues in the polymer and also high catalyst costs needed to produce the polymer. This puts severe requirements on the catalyst because a high catalyst activity is required combined with a polymer particle size below 250 μm, preferably below 200 μnm.
The catalyst must be able to produce UHMWPE with sufficiently high molar mass in an economic efficient way. In processes to produce polyolefin the removal of the heat of polymerization is crucial and consequently the polymerization is carried out at increased temperatures to maximize reactor output per time and to reduce energy costs related to heat removal. Therefore it is desirable to apply a polymerization temperature as high as possible. However at increased polymerization temperatures the Ziegler catalysts tend to produce lower molar mass polymers. So the highest possible temperature that can be applied is affected by the highest molar mass that a certain Ziegler catalyst can produce. So there is a need for catalysts that are able to produce very high molar mass polyethylene at elevated polymerization temperature.